Systems and methods for high-speed, spectroscopic, gas-phase thermometry

ABSTRACT

Systems and methods for measuring temperature in an environment by creating a first beam having an energy of about 50 mJ/pulse, and a pulse duration of about 100 ps. A second beam is also created, having an energy of about 2.3 mJ/pulse, and a pulse duration of about 58 ps. The first beam and the second beam are directed into a probe region, thereby expressing an optical output. Properties of the optical output are measured at a sampling rate of at least about 100 kHz, and temperature measurements are derived from the measured properties of the optical output. Such systems and methods can be used to measure temperature in environments exhibiting highly turbulent and transient flow dynamics.

GOVERNMENT INTEREST

The invention described herein may be manufactured, used, and licensedby or for the Government of the United States for all governmentalpurposes without the payment of any royalty.

PRIORITY CLAIM

This application claims rights and priority on prior U.S. provisionalpatent application Ser. No. 62/404,397 filed 5 Oct. 2016, andnon-provisional patent application Ser. No. 15/582,792 filed 1 May 2017,the entireties of which are incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to the field of temperature measurement. Moreparticularly, this invention relates to coherent anti-Stokes Ramanscattering (CARS) thermometry.

BACKGROUND OF THE INVENTION

CARS spectroscopy has proven to be a valuable tool for measuringtemperature and major species concentrations in gas-phase reactingflows. CARS is a nonlinear diagnostic technique that relies on inducingRaman coherence in the target molecule(s) using two lasers. When thiscoherence is probed by a third laser, a coherent laser-like signal isgenerated in the phase-matching direction at a shifted frequency. Thisshifted signal is analyzed to achieve measurements of gas-phasetemperature and species concentrations in reacting flows.

Over the last four decades, coherent anti-Stokes Raman scattering (CARS)spectroscopy has been widely used for thermometric andspecies-concentration measurements in gas-phase reacting andnon-reacting flows. The spatio-temporally resolved nature of thisapproach, coupled with the coherent laser-like signal, allows forvirtual time-freezing of the measured flow, thereby providing anear-instantaneous and highly accurate snapshot of the localtemperature.

Until recently, gas-phase CARS-based temperature measurements weregenerally performed at rates of from about ten hertz to about twentyhertz. Other techniques have increased the measurements by more than twoorders of magnitude to about five kilohertz, thereby enablingtemporally-correlated, spatially-resolved measurements in moderatelyturbulent reacting flows with moderate Reynolds numbers (Re).

However, these measurement rates are not fast enough to investigateapplications that exhibit highly turbulent and transient flow dynamics.Such applications include (1) high-Reynolds-number subsonic flows, (2)high-enthalpy hypervelocity environments in hypersonic propulsion, (3)high-speed transient thermometry for studying boundary-layerinstability, (4) internal energy-exchange processes of interactingfluids, (5) shock-wave/boundary-layer interactions in high-speed flows,(6) explosion dynamics, and (7) flow dynamics of rocket propulsion.

For example, in high-enthalpy hypervelocity environments (application(2) above), measurements would preferably be performed at rates of aboutone-hundred kilohertz to about five megahertz (or from about twenty toabout one thousand times faster than other techniques can provide).

What is needed, therefore, is a system that tends to address issues suchas those described above, at least in part.

BRIEF SUMMARY OF THE INVENTION

The above and other needs are met by the systems and methods formeasuring temperature as described herein. According to one embodiment,a method for measuring temperature is provided by creating a first beamhaving an energy of about 50 mJ/pulse, e.g. 50-200 mJ/pulse, and a pulseduration of about 100 ps, e.g. 50-200 ps. A second beam is also created,having an energy of about 2.3 mJ/pulse, e.g. 2-7 mJ/pulse, and a pulseduration of about 58 ps, e.g. 55-100 ps. The first beam and the secondbeam are directed into a probe region within an environment, therebyexpressing an optical output from the environment. Properties of theoptical output are measured at a sampling rate of at least about 100kHz, e.g. 100-500 kHz, and temperature measurements are derived from themeasured properties of the optical output.

Various embodiments of the current invention extend the measurement rateof state-of-the-art CARS-based thermometry by at least about twentytimes by using a burst-mode laser architecture having a burst durationof from about one millisecond to about ten milliseconds and acharacteristic pulse width of about 100 ps. The short duration pulsetrain of the burst mode laser is ideal for facilities where temporallycorrelated measurements must be performed within a very limited timewindow.

The developed technique and system allow for the enhancement ofdata-acquisition rate (kHz-MHz CARS), enable investigation of systemsthat exhibit highly turbulent and transient flow dynamics, and providevaluable data for model validation related to high-Reynolds-numbersubsonic flows, high-enthalpy hypervelocity environments forhypersonic-propulsion research, high-speed transient thermometry forstudying boundary-layer instability and the internal energy-exchangeprocesses of interacting fluids, shockwave/boundary-layer interactionsin high-speed flows, explosion dynamics, and flow dynamics related torocket propulsion.

In various embodiments according to this aspect of the invention, thefirst beam has a wavelength of about 532 nm, e.g. 532 nm+/−1.0 nm. Insome embodiments, the second beam has a wavelength of about 680 nm, e.g.680 nm+/−1.0 nm. In some embodiments, the first beam and the second beamhave burst durations of at least about 1 ms, e.g. 1-10 ms. In someembodiments, the second beam is created by directing a portion of thefirst beam into an OPG/OPA. Some embodiments include an OPG/OPA oftype-I beta-barium-borate crystals that are cut at an angle of about 21°(i.e. 21°+/−0.1°) to an optical axis, with a crystal length of about 10mm, e.g. 10-15 mm, and about an 8-10 mm² cross section. In someembodiments, the first laser is created by a Nd:YAG-based picosecondburst-mode laser. In some embodiments, the properties of the opticaloutput are measured with a spectrometer. In some embodiments, themeasurements are recorded with an electron multiplying charge coupleddevice camera. In some embodiments, the optical output is passed througha bandpass filter prior to the step of measuring the properties of theoptical output. In some embodiments, the first beam is a combined pumpand probe beam. In some embodiments, the second beam is a Stokes beam.

According to another aspect of the invention there is described acoherent anti-Stokes Raman scattering spectography system, having aNd:YAG-based picosecond burst-mode laser for producing a combinationpump and probe beam having a wavelength of about 532+/−1.0 nm, an energyof about 50-200 mJ/pulse, e.g. 50 mJ/pulse, and a pulse duration of50-200 ps, e.g. about 100 ps. An OPG/OPA formed of type-Ibeta-barium-borate crystals that are cut at an angle of 21°+/−0.1° to anoptical axis, with a crystal length of 10-15 mm, e.g. about 10 mm, andan 8-10 mm² cross section, receives a portion of the combination pumpand probe beam and thereby produces a Stokes beam having a wavelength of680+/−1.0 nm, an energy of 2-7 mJ/pulse, e.g. about 2.3 mJ/pulse, abandwidth of 150-250 cm⁻¹, e.g. about 200 cm⁻¹, and a pulse duration of55-120 ps, e.g. about 58 ps. Optics direct the probe beam and the Stokesbeam into a common probe region within an environment for a duration of1-10 ms, e.g. about 1 ms, thereby expressing an optical output from theenvironment. A spectrometer receives the optical output and produces aspectrum.

A number of operational parameters have been presented as ranges ofvalues as well as specific values within the defined ranges. Theoperation of the spectrography system described herein is fullyoperational within the defined ranges, as well as any subset of thedefined ranges.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages of the invention are apparent by reference to thedetailed description when considered in conjunction with the figures,which are not to scale so as to more clearly show the details, whereinlike reference numbers indicate like elements throughout the severalviews, and wherein:

FIG. 1 is a functional block diagram of a CARS system, according to anembodiment of the present invention.

FIG. 2A is graph of a burst profile for a pump beam with a onemillisecond duration, a frequency of 100 kilohertz, and a wavelength of532 nanometers, according to an embodiment of the present invention.

FIG. 2B is a graph of a burst profile for a probe beam that correspondsto the output of an OPG/OPA with a wavelength of 680 nanometers,according to an embodiment of the present invention.

FIG. 2C is a graph of the OPG/OPA beam spectra at 100 kilohertz, showingevery tenth spectrum, according to an embodiment of the presentinvention.

FIG. 2D is a graph of the OPG/OPA output energy as a function ofpump-pulse energy, according to an embodiment of the present invention.

FIG. 3A is a graph of a single-shot picosecond-CARS hydrogen spectrumrecorded in an hydrogen/air flame that is stabilized over a Henckenburner at q=5.2, according to an embodiment of the present invention.

FIG. 3B is a graph of an integrated, single-shot, picosecond-CARShydrogen signal as a function of time, according to an embodiment of thepresent invention.

FIG. 3C is a graph of a corresponding temperature for fifty consecutivelaser shots, according to an embodiment of the present invention.

FIG. 4A is a graph of temperatures that have been extracted from aspectrograph of fifty consecutive single-shot, picosecond-CARS hydrogenspectra taken at 100 kilohertz from a turbulent flame, according to anembodiment of the invention.

FIG. 4B is a graph of temperature probability density functions (PDFs)for ten bursts (a total of 500 laser shots), according to an embodimentof the invention.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention enable coherent anti-Stokes Ramanscattering (CARS) spectroscopy to be performed at kilohertz (kHz) ratesthrough megahertz (MHz) rates, such as above about 100 kHz. CARSspectroscopy at these rates makes possible temperature measurements inenvironments exhibiting highly turbulent and transient flow dynamics,such as may be present in an environment, such as a combustingenvironment, for example. Typical CARS temperature accuracy is aboutplus or minus two percent.

Operational Overview

Generalized embodiments of the present invention can include (1) aburst-mode laser, (2) a broadband laser, and (3) a high-speed spectrallyresolved CARS-signal detection system. In some embodiments, an OPG/OPAis implemented as the broadband laser, because of general limitations inthe current state of alternate technologies.

With reference now to FIG. 1, there is depicted a CARS spectroscopysystem 10 according an embodiment of the present invention. The systemincludes a burst-mode laser 12 for producing a beam 56 directed along afirst path 13, defined as the path between the burst mode laser 12 andthe beam dump 26. Burst-mode laser 12 is, in some embodiments, ahigh-energy, high-repetition-rate picosecond burst-mode laser, such as aNd:YAG-based laser, that in some embodiments produces a generally greenlaser beam 56 with a diameter of about 6 mm and having an energy ofabout 50 mJ/pulse and a wavelength of about 532 nm. Beam 56 is, in someembodiments, nearly transform-limited with a full-width at half-maximum(FWHM) pulse duration of about 100 picoseconds (ps) and a measuredlinewidth of about 0.2 cm⁻¹.

The beam 56 encounters a beam splitter 14, which passes a portion of thebeam 56 along the first path 13, and diverts another portion of the beam56 along a second path 17, which is defined between the beam splitter 14and the spectrometer 46. The portion of the beam 56 that is divertedalong the second path 17 is, in some embodiments, used as a probe beam56. The portion of the beam 56 that is passed through the beam splitter14 is, in some embodiments, used to produce a Stokes beam 60, all asdescribed in more detail hereafter.

The portion of the beam 56 that is passed by the beam splitter 14continues along the first path 13 through conditioning optics 16, suchas a half-wave plate 16 a, a concave lens 16 b, and convex lens 16 cthat in some embodiments reduce the beam 56 from about a 6 mm diameterto about a 3 mm diameter. The reduced and conditioned beam 56 thenpasses through a solid-state, broadband, wavelength-tunable device,which in some embodiments is an optical parametric generator/opticalparametric amplifier (OPG/OPA) 18/20, which produces as an output theStokes beam 60, which also passes along a portion of the first path 13.

Some embodiments of the OPG/OPA 18/20 use type-I beta-barium-borate(BBO) crystals that are cut at an angle of about 21° to the opticalaxis. In some embodiments the crystal length is about ten millimeters,with about an 8-mm² cross section.

In the embodiment depicted, the burst-mode laser 12 produceshigh-energy, transform-limited picosecond laser pulses 56 at arepetition rate of about 100 kHz or higher, providing sufficientpicosecond pulse energy to pump the OPG/OPA 18/20, and generate ahigh-energy broadband Stokes laser beam 60.

In some embodiments, the Stokes beam 60 is a generally red beam with awavelength of about 680 nm, a FWHM bandwidth of about 8 nm, a pulseduration of about 58 ps, a bandwidth of about 200 cm⁻¹, and a pulseenergy of about 2.3 mJ.

In the illustrated embodiment, the Stokes beam 60 and residual portionsof the beam 56 exit the OPG/OPA 18/20 and pass along the first path 13through additional conditioning optics 22, before encountering anotherbeam splitter 24. The beam splitter 24, in some embodiments, allows theresidual portions of the beam 56 to pass through along the first path 13to a beam dump 26. The beam splitter 24 diverts the Stokes beam 60 alonga third path 19, and through another set of conditioning optics 28, suchas a lens 28 a, bandpass filter 28 b, and half wave plate 28 c. Afterpassing through the conditioning optics 28, the Stokes beam 60 is foldedby a mirror 30 to continue along the third path 19.

Returning now to the probe beam 56 that was diverted by the beamsplitter 14 along the second path 17, it is delivered along the secondpath 17 to the folding mirror 30. However, the second path 17 betweenthe beam splitter 14 and the mirror 30 includes some delay elements,such as might include a longer path length, using mirrors 50 and 54, andprism 52, such that it leaves mirror 30 in phase with the Stokes beam60, which was delayed by the OPG/OPA 18/20.

Some characteristics of the Stokes beam 60 are depicted in FIG. 2.Typical burst profiles showing individual pulse energy for the pump beam56 (about 532 nm) and the corresponding Stokes beam 60 (in this case, aStokes beam having a wavelength of about 680 nm) over about a onemillisecond burst duration with a frequency of about 100-kHz aredisplayed in FIGS. 2A and 2B, respectively. The pulse-to-pulse standarddeviation across the burst (after reaching a plateau) is about 4% andabout 6% for the pump beam 56 and the Stokes beam 60, respectively. Thedynamic spectral profile for the Stokes beam 60 for a few pulses isdisplayed in FIG. 2C. The recorded pulse-to-pulse variation of thespectral width is about 5%. The dependence of the pulse energy of theStokes beam 60 on the pulse energy of the pump beam 56 is depicted inFIG. 2D.

From a power-law fit it is observed that the intensity of the Stokesbeam 60 is proportional to the pulse energy of the pump beam 56 raisedto the power of about 4.87. In some embodiments the maximum parametricdown-conversion efficiency reaches about 5%. The conversion efficiencyis limited in some embodiments by the threshold of about 1.8 GW/cm² atwhich the coating of the OPG/OPA 18/20 is damaged. For burst durationsthat are longer than about one millisecond, a small deterioration ofefficiency is observed in some embodiments, which might be attributed toheating of the OPG/OPA 18/20 and the resulting change of thephase-matching conditions.

The probe beam 56 and the Stokes beam 60 are directed onto and folded bythe mirror 30 in such a manner that the second path 17 and the thirdpath 19 are axial, one to another, after leaving the mirror 30. Thebeams 56 and 60 then encounter, in some embodiments, a spherical lens 32with a focal length of about 200 mm, at a relatively small incidentangle of about 3°, to achieve a desirable phase-matching condition forall of the transitions covered by the bandwidth of the OPG/OPA 18/20. Insome embodiments, the spherical lens 32 crosses the probe beam 56 andthe Stokes beam 60 at a measurement point within a probe region 58,which is located within an environment where temperature is to bemeasured.

The interaction of the probe beam 56 and the Stokes beam 60 produces, inthe embodiment described herein, a hydrogen CARS signal that isindicative of, among other things, the temperature at the precise pointin the probe region 58 as which the probe beam 56 and the Stokes beam 60interacted, and at the exact time that the synched pulses of the probebeam 56 and the Stokes beam 60 interacted. This CARS signal is carriedalong the second path 17 with the probe beam 56.

In the illustrated embodiment, probe region 58 may be a gaseous reactionregion, such as a flame, emitting from a burner or engine 34, where anadditional electron that is characteristic of the temperature of theflame 58 is emitted along the path of the probe beam 56. In someembodiments, the probe beam 56 (including the CARS signal) and theStokes beam 60 are conditioned by lens 36 such that the second path 17and the third path 19 are once again axial, one to another.

The Stokes beam 60 is then absorbed by a beam dump 38, in someembodiments, and the probe beam 56 (with the CARS signal) is directed bymirror 40 through conditioning optics 42, which in some embodimentsinclude a focusing lens 42 b to focus the combined components of beam 56into the spectrometer 46. Conditioning optics 42 in some embodimentsinclude an 8 nm bandpass filter 42 b centered at about 439 nm (such asSemrock, LD01-439/8-25.4), to minimize signal interference from flameemission and laser scatter.

The probe beam 56 and resulting hydrogen CARS signal is accepted by ahigh-speed spectrally resolved CARS-signal detection system 46, such asa 0.25-m spectrometer 46 that is equipped with a 2,400 groove/mmgrating. The output of the spectrometer 46 is observed by a recordingdevice 48, such as a high-speed EMCCD camera 48 (such as a PrincetonInstruments ProEM-HS 1024BX3) with a 1024×1024 pixel array, which isused in some embodiments for the acquisition of the CARS spectra at arate of about 100-kHz, and is phase-locked to be synchronized with thelaser 12. In this embodiment, the camera 48 chip is illuminated overabout 10 pixels (which is equivalent to a frame-shifting time of about 8microseconds) to achieve the signal-acquisition speed of about 100 kHz.The camera 48 chip size is about 13×13 microns. This high-speed EMCCDcamera 48 can operate at up to about 333 kHz. For data-acquisitionspeeds greater than 333 kHz, the camera 48 could be replaced with aFASTCAM SA-Z (from Photron).

The output of the recording device 48 is input to an analyzer 62, wherethe CARS signal is analyzed to determine properties of the environment58, such as temperature, according to known principles. The CARS signalis temperature dependent. The strength of the signal scales non-linearlywith the difference in the ground state population and the vibrationallyexcited state population. Since the population of these states followsthe temperature-dependent Boltzmann Distribution, the CARS signalcarries an intrinsic temperature dependence as well.

In actual tests, single-laser-shot temperature measurements wereperformed in highly turbulent hydrogen/nitrogen jet-diffusion flames 58at rates of 100 kHz. The Reynolds number for the jet flame 58, based ona diameter of about 9.5 mm for the jet flame 58, was about 10,000. Themeasurements were made at a distance of about 50 mm from the jet 34exit. Information in regard to the measurements is discussed below, withreference to FIGS. 3 and 4.

Measurements were initially performed in an atmospheric-pressure, nearlyadiabatic hydrogen/air flame 58 that was stabilized over a Henckenburner 34. The burner 34 was operated at high flow rates to minimizeheat losses to the burner 34. The energies used for pump 56 and Stokes60 pulses were about 2 mJ/pulse and about 2.3 mJ/pulse, respectively.FIG. 3A displays a single-shot spectrum for hydrogen at an equivalenceratio (ϕ) of 5.2. The Q-branch transitions of the v′=1→v″=0 vibrationalband of hydrogen are shown in FIG. 3A. The extracted temperatureT_(CARS)=1410 K agrees well with the adiabatic flame temperatureT_(adiab)=1412 K.

FIG. 3B displays typical integrated CARS signal as a function of time.The CARS signal becomes stable after the sixteenth pulse of the pulsetrain, and this signal level is maintained from Frame 15 to Frame 65 (50measurement data points, which is equivalent to a measurement time of0.5 ms). The corresponding temperature acquired from each single-shotspectrum is shown in FIG. 3C.

Depicted in FIG. 4A are temperature values derived from the hydrogenpicosecond-CARS signal that was obtained in the turbulent flame during aone millisecond burst. A rapid temperature change in the turbulent flamecan be observed. Probability-density functions (PDFs) of temperaturederived from 500 single-shot spectra collected from ten bursts aredepicted in FIG. 4B. The obtained mean temperature and standarddeviation of the single-shot thermometry in turbulent flames 58 were 870K and 410 K, respectively. The jet flame 58 has a very wide temperaturedistribution because of turbulent mixing of the fuel (hydrogen) and theoxidizer (ambient air).

The foregoing description of embodiments for this invention has beenpresented for purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formdisclosed. Obvious modifications or variations are possible in light ofthe above teachings. The embodiments are chosen and described in aneffort to provide illustrations of the principles of the invention andits practical application, and to thereby enable one of ordinary skillin the art to utilize the invention in various embodiments and withvarious modifications as are suited to the particular use contemplated.All such modifications and variations are within the scope of theinvention as determined by the appended claims when interpreted inaccordance with the breadth to which they are fairly, legally, andequitably entitled.

The invention claimed is:
 1. A method for measuring gas phasetemperature in an environment, the method comprising: creating a firstbeam having an energy of 50-200 mJ/pulse, and a pulse duration of 50-200ps; creating a second beam having an energy of 2.3-2.7 mJ/pulse, and apulse duration of 55-100 ps; directing the first beam and the secondbeam into a probe region within the environment, thereby expressing anoptical output from the environment; channeling the optical outputthrough an 8 nm bandpass filter centered at 439 nm prior to measuringthe properties of the optical output; measuring properties of theoptical output at a sampling rate of at least 100-500 kHz; and derivingthe temperature from the measured properties of the optical output. 2.The method of claim 1, wherein the first beam has a wavelength of532+/−1 nm.
 3. The method of claim 1, wherein the second beam has awavelength of 680+/−1 nm.
 4. The method of claim 1, further comprisingchanneling the optical output through a bandpass filter prior tomeasuring the properties of the optical output.
 5. The method of claim1, wherein the first beam and the second beam are created for a burstduration of at least 1-10 ms and a repetition rate of at least 100 kHz.6. The method of claim 1, wherein the second beam is created bydirecting a portion of the first beam into an optical parametricgenerator/optical parametric amplifier (OPG/OPA).
 7. The method of claim1, wherein the second beam is created by directing a portion of thefirst beam into an optical parametric generator/optical parametricamplifier (OPG/OPA) of type-I beta-barium-borate crystals that are cutat an angle of about 21°+/−1° to an optical axis, with a crystal lengthof 10-15 mm, and an 8-10 mm² cross section.
 8. The method of claim 1,wherein the first beam is created by a picosecond burst-mode laser. 9.The method of claim 1, wherein the properties of the optical output aremeasured with a spectrometer.
 10. The method of claim 1, furthercomprising recording measurements with an electron multiplying chargecoupled device camera.
 11. The method of claim 1, wherein the first beamis configured to be a combined pump and probe beam, the second beam isconfigured to be a Stokes beam, and the second beam is derived from thefirst beam.
 12. The method of claim 1, wherein the second beam is aStokes beam.
 13. A method for measuring temperature in an environment,the method comprising: creating a first beam with a picosecondburst-mode laser, the first beam having a wavelength of 532+/−1 nm, anenergy of 50-200 mJ/pulse, and a pulse duration of 50-200 ps; directinga portion of the first beam into an optical parametric generator/opticalparametric amplifier (OPG/OPA), thereby creating a second beam having awavelength of 680+/−1 nm, an energy of 2.3-2.7 mJ/pulse, a bandwidth of150-250 cm⁻¹, and a pulse duration of 55-100 ps; directing the firstbeam and the second beam into a probe region within the environment fora pre-determined duration, thereby expressing an optical output from theenvironment; channeling the optical output through an 8 nm bandpassfilter centered at 439 nm prior to measuring the properties of theoptical output; measuring properties of the optical output at a samplingrate of at least 100 kHz; and deriving the temperature of theenvironment from the measured properties of the optical output.
 14. Themethod of claim 13, wherein the OPG/OPA is formed of type-Ibeta-barium-borate crystals that are cut at an angle of 21°+/−1° to anoptical axis, with a crystal length of 10-15 mm, and an 8-10 mm² crosssection.
 15. The method of claim 13, wherein the properties of theoptical output are measured with a spectrometer.
 16. The method of claim13, further comprising recording measurements with an electronmultiplying charge coupled device camera.
 17. The method of claim 13,wherein the first beam is a combined pump and probe beam.
 18. The methodof claim 13, wherein the second beam is a Stokes beam.